Corrections de quelques warnings php.

git-svn-id: http://piwigo.org/svn/branches/release-1_3@274 68402e56-0260-453c-a942-63ccdbb3a9ee

admin/admin.php

@@ -35,6 +35,7 @@ if ( is_file( './install.php' ) )
 //--------------------------------------- validating page and creation of title
 $page_valide = false;
 $title = '';
+if (isset( $_GET['page'] ))
 switch ( $_GET['page'] )
 {
  case 'user_list':